<!doctype html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
<script>
	var str = 'aa';
	alert(str.sub());

	var str="abc"
	alert(str.sub());		//	<sub>abc</sub>
</script>
</body>
</html>